Method for producing high purity copper castings



Oct. 7. 1969 H. L. JANDRAS 3,470,936

METHOD FOR PRODUCING HIGH PURITY COPPER CASTINGS Filed Sept. 12, 1966 CHARGE CRUCIBLE WITH HIGH CONDUCTIVITY PIG AND SCRAP COPPER PLACE CRUCIBLE AND RECEIVING MOLD IN VACUUM TANK EVACUATE VACUUM TANK TO ABOUT 0.0I MM OF MERCURY HEAT CRUCIBLE TO BETWEEN ABOUT I900F TO 2000F TO MELT COPPER AND INTRODUCE SUFFICIENT QUANTITY OF INERT GAS TO OBTAIN A VACUUM OF ABOUT 50 MM TO ABOUT 250 MM OF MERCURY II INTERRUPT THE FLOW OF INERT GAS AND v REFINE COPPER BY RAISING TEMPERATURE TO BETWEEN ABOUT 2IOOF TO ABOUT ,ZBOO'F HOLD FOR. ABOUT I HR. TO ABOUT 8 HRS.

. I INTRODUCE A SUFFICIENT QUANTITY OF INERT GAS TO'OBTAIN A VACUUM OF 50 MM TO ABOUT 600 MM OF MERCURY AND CAST MOLTEN METAL WITHIN TEMPERATURE RANGE OF ABOUT 2I50F TO ABOUT 2800F MAINTAIN INERT ATMOSPHERE AND VACUUM WHILE MOLTEN METAL SOLIDFIES AT A RATE OF ABOUT 30F/HR. TO ABOUT 200F/HR.

REMOVE MOLD FROM TANK AND "SHAKE-OUT" CASTING AT A TEMPERATURE OF ABOUT 200F INVENTOR Henry L. Jondros United States Patent ABSTRACT OF THE DISCLOSURE Method for producing a high purity copper casting including melting charged raw materials in a crucible in a vacuum tank in a desired vacuum at a desired temperature and in a desired atmosphere of an inert gas, superheating the molten bath for a time sufiicient to refine said bath 5 Claims I under a desired vacuum, teeming said bath into a suitable mold within the tank under a desired vacuum and desired atmosphere and cooling the cast metal in the tank at a desired rate under a desired vacuum in a desired atmosphere to obtain a solidified casting. The copper casting so produced is characterized by having improved cold working characteristics and improved resistance'to corrosion.

This invention relates to a method for producing high purity copper castings having improved cold working characteristics and resistance to corrosion.

Present art practice of producing copper in a vacuum process requires that the crucible in which the copper is to be melted and the mold into which the molten copper is to be cast be made of a material inert to copper, for example, zirconium 'or magnesia. The'crucible and casting mold are'preheated and outgassed prior to charging the materials to be melted. The molten metal in the mold is cooled rapidly to produce a solidified casting.

, Castings made by the above method are characterized by shrinkage problems and cavities caused by entrapped gases. Little or no improvement over copper castings produced by air melt and poured means is obtained in the cold working characteristics or in the corrosion resistance of the castings. t

- It is therefore the object of this invention to produce high puritycopper castings having improved cold working characteristics and resistance to corrosion by a vacuum process which does not require a crucible or a mold made of an inert material, said copper castings being slow cooled in the mold. The drawing is a diagrammatic 'repre-{ sentation of the method of the invention.

Broadly, the invention includes charging a crucible with solid charge materials in a vacuum induction'furnace,

melting the charge materials under theinfluenceof a vacuum, in an inert atmosphere, superheating the molten bath under the influence'of a vacuum for a time sufficient to refine said bath, casting the refined bathinto a casting mold under the influence of a vacuum in an inert atmosphere and cooling the cast metal at a prescribed rate to obtain a solidified casting.

In a more detailed description of the invention, high conductivity copper and/ or scrap copper analyzing 99.8% copper, the remainder being small amounts of impurities such as lead, tin, arsenic and iron, is charged into a crucible, said crucible being made of any suitable material, for example graphite, in a vacuum induction furnace in a vacuum tank. A suitable casting mold, for example a dry sand mold, is placed inside the vacuum tank to receive the melted refined metal. The charge materials are melted at a temperature of about 1990 F. to about 2100 F. under the influence of a vacuum of between 50 mm. and 250 mm. of mercury, but I prefer a vacuum of 50 mm. An inert gas, for example, argon or nitrogen is introduced into ice the vacuum tank to suppress. the reaction of the molten copper under the vacuum and to form an inert atmosphere in the vacuum tank.

After the charged copper is melted, the temperature of the molten metal is raised to betwen 2100 F. and 2800 F. for the refining step. It is preferred to raise the temperature to about 2150 F. The molten metal is held within this temperature range for from 1 hour to 8 hours. I prefer about 3 hours. The vacuum in the vacuum tank is kept to 0.01 mm. of mercury during the refining step. Gases which have been entrapped in the copper are removed during this step as is a portion of the minor elements, such as lead, contained therein.

The refined molten metal is cast into a suitable mold as previously mentioned. The vacuum in the furnace may be within the range of 50 mm. to 600 mm. of mercury during the solidification of the molten copper in the mold. I have found that a vacuum of 300 mm. of mercury suppresses the reaction of the copper in said casting mold. Casting temperatures may range from 2150 F. to 2800 F. I prefer to cast the molten metal within the temperature range of 2150 F. to about 2355' F. An inert gas for example argon or nitrogen is intermittently introduced into the vacuum tank during the casting step and during solidification of the molten copper.

In prior art practices, it has been customary to rapidly cool the molten metal in the mold rather than retard the coolingof the molten metal. However, I have found that by cooling the molten metal in the mold at a prescribed relatively slow rate of from 30 F./hour to about 200 F ./hour, and preferably at a rate of about 140 F./

' hour, under the influence of a vacuum in an inert atmosphere, castings which are free of shrinkage characteristics and porosity are produced. The solidified castings are more dense than castings produced by other, conventional means and have improved cold Working characteristics and improved resistance to corrosion.

In a specific example of the invention, 300 pounds of high conductivity pig copper having the following analysis= by weight:

.the remainder incidental amounts of other elements, was charged'into' a graphite crucible in a vacuuminduction furnace in a vacuum tank. The vacuum tank was pumped down to obtain a vacuum. As the vacuum was beingformed full power wasv placed on. the induction furnace to start melting the charged copper. A vacuum of 0.01 mm. of mercury was formed after 1 hour of pumping. A temperature of 2090 F. was obtained in the furnace. After three hours at this temperature, the charge copper was molten. In order to control the boiling action of the molten copper, argon was introduced into the vacuum tank and a vacuum of 50 mm. of mercury was formed. The temperature of the molten copper was raised to 2130" F. and maintained for 3 hours to obtain maximum degassing of the molten copper. No argon was introduced into the vacuum tank during refining. The vacuum was increased to 0.01 mm. of mercury. After 3 hours, the power was interrupted and argon was introduced into the vacuum tank to obtain a vacuum of 600 mm. of mercury. The molten copper was teemed into a chill mold in the vacuum tank. The molten copper in the mold was allowed to cool at a rate of 138 F./hour in the argon filled vacuum tank until shake-out at which time the castings were at about 200 F. Chemical analysis by weight of the solidified casting was as follows:

Cu 99.8 Pb .0022

Fe .003 Sb 0002 Te .0007 Se .0010 As .0003

A portion of the lead was distilled off during refining, the the remaining elements being largely unatfected.

Specimens poured from the above vacuum melted copper and specimens of conventionally melted phosphorus deoxidized copper and high conductivity copper were subjected to standard ASTM B3 68-65 acetic acid-salt spray corrosion testing procedures. The results are listed in the following table:

ASTM B368-65-ACETIC ACID, SALT SPRAY [Thirty day salt corrosion test (720 hours) (spray, pH 6.54.5, Sp. gr.

Initial Final Loss in weight weight weight Specimen (grams) (grams) Grams Percent Phosphorus deoxidized copper 475. 472. 50 2. 5O 53 High conductivity copper- 473. 25 471. 50 1. 75 37 Vacuum melted copper 458. 50 458. 00 0.50 11 It will be noted that the vacuum melted copper of the invention was about 3.5 times as resistant to corrosion as the high conductivity copper and 5 times as resistant to corrosion as the phosphorus deoxidized copper.

The hardness of the vacuum melted copper increased more rapidly than the phosphorus deoxidized copper and the high conductivity copper wen subjected to 15% cold reduction as noted below:

Hardness Percent Initial (R 151) after increase hardness 15% cold in Specimen (R 15T) reduction hardness Phosphorus deoxidlzed copper 63 74 17. 4

High conductivity copper. 57 63 10. 5

Vacuum melted copper 50 67 34. 0

-I claim:

1. A method for manufacturing a high purity copper casting having a chemical composition of 99.8% cop per by weight, the remainder incidental impurities comprising:

(a) placing a crucible containing a charge of high conductivity copper and scrap copper and a mold into a vacuum tank,

(b) evacuating said tank for a time suflicient to produce a vacuum of about 0.01 mm. of mercury therein,

(c) heating said charge within a temperature range of from about 1900 F. to about 2000 F. and introducing an amount of an inert gas sufficient to reduce the vacuum in said tank to between about 50 mm. to about 250 mm. of mercury,

(d) superheating the molten copper within a temperature range of about 2100 F. to about 2800 F. while interrupting the flow of said inert gas to reestablish a vacuum of about 0.01 mm. of mercury,

(e) holding the molten bath within the superheating temperature range and vacuum of step (d) for about one hour to about eight hours to remove the entrapped gases therefrom and to distill off minor chemical elements,

(f) introducing a suificient amount of an inert gas into said tank to reduce the vacuum to between about 50 mm. to about 600 mm. of mercury,

(g) pouring the molten copper into the mold of step (h) cooling the copper in the mold of step (a) to a temperature of about 200 F. at a rate of about 30 F./hour to about 200 F./hour while maintaining the vacuum and atmosphere of step (f).

2. A method as claimed in claim 1 in which the inert atmosphere of steps (a), (d) and .(e) is argon.

3. A method as claimed in claim 1 in which the temperature of superheating in step (b) is 2130 F.

4. A method as claimed in claim 1 in which the time of holding at temperature in step (c) is about 3 hours.

5. A method as claimed in claim 1 in which the cooling rate of step (e) is 140 F./hour.

References Cited UNITED STATES PATENTS 2,036,496 4/1936 Randolph 164-68 X 2,264,289 12/1941 Betterton et a1. l6468 OTHER REFERENCES Vacuum Melting and Casting of Copper by Stauifer, Fox and DiPietro, Industrial and Engineering Chemistry, pp. 820-825, vol. 40, No. 5, May 1948.

J. SPENCER OVERHOLSER, Primary Examiner V. RISING, Assistant Examiner U.S. Cl. X.R. 

